1. Field of the Invention
The present invention relates to a semiconductor laser element and a manufacturing method for the same. In particular, the present invention relates to a high power semiconductor laser element that is utilized for a CD-R/RW, a DVD-R/RW and the like, and to a manufacturing method for the same.
Usage of a so-called air ridge structure from among the structures of high power semiconductor lasers has been becoming widespread as a way to reduce the cost of materials and in order to reduce the number of steps in the manufacturing process.
The present invention can provide a semiconductor laser element where the adhesion of the metal electrode layer in the ridge portion has been increased so that the efficiency of heat release and the temperature properties are improved in an air ridge structure.
2. Description of the Background Art
FIGS. 7A to 7D are schematic perspective views showing a manufacturing method for a GaAs/GaAlAs based infrared red laser element according to the prior art.
First, as shown in FIG. 7A, a buffer layer 2, a first N type GaAlAs clad layer 3, a second N type GaAlAs clad layer 4, an active layer 5, a first P type GaAlAs clad layer 6, a GaAs etching stopper layer 7, a second P type GaAlAs clad layer 8, and a P type GaAs cap layer 9 are layered in this order on an N type GaAs substrate (wafer) 1, where each of the layers is grown by means of a vapor deposition method such as MOCVD. Here, though in the figures an individual semiconductor laser element is shown, in practice manufacture is carried out in wafer units.
Next, as shown in FIG. 7B, a mask 10 for the formation of a ridge (current path) is provided on the P type GaAs cap layer 9. A material that is resistant to an etching method used is utilized for the mask. Here, in the case of dry etching, a mask made of a film such as a SiO2 film that is resistant to dry etching, is used as the mask for the formation of a ridge.
Next, as shown in FIG. 7C, the entirety of the P type GaAs cap layer 9 is etched and the second P type GaAlAs clad layer 8 is etched up to the vicinity of the GaAs etching stopper layer 7 by means of a dry etching or wet etching technique so as to create a ridge in rough form (this etching is referred to as first etching). Here, this ridge becomes a current path for laser oscillation.
Subsequently, as shown in FIG. 7D, the second P type GaAlAs clad layer 8 is further etched by HF, which is an etchant that can etch only the second P type GaAlAs clad layer 8 and does not etch GaAs (this etching is referred to as second etching). This etching intend to the width of the ridge so that desired laser properties can be gained. In this case, etching by HF is naturally stopped by the GaAs etching stopper layer 7, and therefore, the width of the ridge depends on the length of the period of time of etching.
Next, a P side electrode is formed in a sequential process shown in FIGS. 8A to 8G. In the following, the process is described in reference to FIGS. 8A to 8G.
First, in order to prevent a current from flowing on the surfaces of both sides of the ridge, first a dielectric film (SiN, SiO2 or the like) 11 having insulating property is formed on the wafer surface, including the entire surface of the ridge, so as to have a thickness of approximately 1,000 Å to 2,000 Å (FIG. 8A). Here, the dielectric film 11 also has the effect of stabilizing NFP (near field pattern) at the time of laser beam emission.
Next, the portion of the cap layer other than the top portion is protected by a resistor 12 (FIG. 8B).
Next, only the dielectric film 11 is etched and removed from the top of the P type GaAs cap layer 9 (FIG. 8C) in order to allow a current to flow through the inside of the ridge only. At this time, the dielectric film 11 on both sides of the P type GaAs cap layer 9 is partially over-etched as shown in FIG. 8C.
Furthermore, a first metal electrode layer 13 is formed of AuZn in order to contact the P type GaAs cap layer 9 to a thick film electrode of gold in an ohmic condition (FIG. 8D).
After this, the resistor 12 is removed (FIG. 8E) and a second metal electrode layer (barrier/die bonding electrode) 14 is formed of Mo/Au (FIG. 8F). At this time, as shown in FIG. 8F, the second metal electrode layer 14 is formed only on the dielectric film 11 on both sides of the P type GaAs cap layer 9. This is because it is difficult for the second metal electrode layer 14 to be formed in the vicinity of the steps (portions directly beneath eaves, i.e. the sides of the top layers) formed between the dielectric film 11 and both sides of the P type GaAs cap layer 9.
Next, a thick film electrode 16 of gold is formed on the wafer surface, including the ridge, by means of plating so as to have a thickness of approximately 2 μm to 3 μm (FIG. 8G). The thick film electrode 16 is formed by means of plating because a current flows from the surface of the second metal electrode layer 14, and thereby, the thick film electrode 16 can be grown with coverage better than by a deposition method.
After this, the N substrate side (bottom side) of the wafer is polished so as to adjust the wafer to have a desired thickness, and then an N side electrode is formed on the N substrate side so that a laser wafer 18, where a number of laser elements are formed as shown in FIG. 9, is completed.
Next, as shown in FIG. 9, the laser wafer 18 is divided into bars 19 having a predetermined width corresponding to the length of the resonator. After this, a protective film having a predetermined reflectance ratio is formed on both edge surfaces for emitting light, and each bar 19 is divided into individual laser elements (chips) (not shown).
Here, a semiconductor laser element that can be gained according to the same method as above is also illustrated in Japanese Unexamined Patent Publication No. 2003-86902.
In addition, a semiconductor laser element that can be gained according to the similar method as above, though it does not have an air ridge structure, is also illustrated in Japanese Unexamined Patent Publication No. HEI 11(1999)-135884.
The P type GaAs cap layer 9 has portions 9′ located beneath the P type GaAs cap layers 9 as shown in the above described FIG. 8F, according to the prior art. The lower portions 15 of these portions 9′ are behind the top layers at the time of the formation of the second metal electrode layer 14, and therefore, the second metal electrode layer 14 becomes much thinner than other portions or is not formed in the lower portions 15. In this case, it becomes difficult for the thick film electrode 16 to be formed by means of plating on regions where the second metal electrode layer 14 is thin or does not exist, and therefore, cavities 17 occur, as shown in FIG. 8G.
These cavities are air layers, and it becomes difficult for the heat generated at the time of laser oscillation to be released due to these cavities, and thus the temperature properties as well as the reliability of the laser element are deteriorated.